[clang][ScanDeps] Canonicalize -D and -U flags (#82298)

Canonicalize `-D` and `-U` flags by sorting them and only keeping the
last instance of a given name.

This optimization will only fire if all `-D` and `-U` flags start with a
simple identifier that we can guarantee a simple analysis of can
determine if two flags refer to the same identifier or not. See the
comment on `getSimpleMacroName()` for details of what the issues are.

clang/include/clang/Tooling/DependencyScanning/DependencyScanningService.h

@@ -60,7 +60,10 @@ enum class ScanningOptimizations {
   /// Remove unused -ivfsoverlay arguments.
   VFS = 4,
 
-  DSS_LAST_BITMASK_ENUM(VFS),
+  /// Canonicalize -D and -U options.
+  Macros = 8,
+
+  DSS_LAST_BITMASK_ENUM(Macros),
   Default = All
 };
 

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -179,6 +179,78 @@ static void sanitizeDiagOpts(DiagnosticOptions &DiagOpts) {
   DiagOpts.IgnoreWarnings = true;
 }
 
+// Clang implements -D and -U by splatting text into a predefines buffer. This
+// allows constructs such as `-DFඞ=3 "-D F\u{0D9E} 4 3 2”` to be accepted and
+// define the same macro, or adding C++ style comments before the macro name.
+//
+// This function checks that the first non-space characters in the macro
+// obviously form an identifier that can be uniqued on without lexing. Failing
+// to do this could lead to changing the final definition of a macro.
+//
+// We could set up a preprocessor and actually lex the name, but that's very
+// heavyweight for a situation that will almost never happen in practice.
+static std::optional<StringRef> getSimpleMacroName(StringRef Macro) {
+  StringRef Name = Macro.split("=").first.ltrim(" \t");
+  std::size_t I = 0;
+
+  auto FinishName = [&]() -> std::optional<StringRef> {
+    StringRef SimpleName = Name.slice(0, I);
+    if (SimpleName.empty())
+      return std::nullopt;
+    return SimpleName;
+  };
+
+  for (; I != Name.size(); ++I) {
+    switch (Name[I]) {
+    case '(': // Start of macro parameter list
+    case ' ': // End of macro name
+    case '\t':
+      return FinishName();
+    case '_':
+      continue;
+    default:
+      if (llvm::isAlnum(Name[I]))
+        continue;
+      return std::nullopt;
+    }
+  }
+  return FinishName();
+}
+
+static void canonicalizeDefines(PreprocessorOptions &PPOpts) {
+  using MacroOpt = std::pair<StringRef, std::size_t>;
+  std::vector<MacroOpt> SimpleNames;
+  SimpleNames.reserve(PPOpts.Macros.size());
+  std::size_t Index = 0;
+  for (const auto &M : PPOpts.Macros) {
+    auto SName = getSimpleMacroName(M.first);
+    // Skip optimizing if we can't guarantee we can preserve relative order.
+    if (!SName)
+      return;
+    SimpleNames.emplace_back(*SName, Index);
+    ++Index;
+  }
+
+  llvm::stable_sort(SimpleNames, [](const MacroOpt &A, const MacroOpt &B) {
+    return A.first < B.first;
+  });
+  // Keep the last instance of each macro name by going in reverse
+  auto NewEnd = std::unique(
+      SimpleNames.rbegin(), SimpleNames.rend(),
+      [](const MacroOpt &A, const MacroOpt &B) { return A.first == B.first; });
+  SimpleNames.erase(SimpleNames.begin(), NewEnd.base());
+
+  // Apply permutation.
+  decltype(PPOpts.Macros) NewMacros;
+  NewMacros.reserve(SimpleNames.size());
+  for (std::size_t I = 0, E = SimpleNames.size(); I != E; ++I) {
+    std::size_t OriginalIndex = SimpleNames[I].second;
+    // We still emit undefines here as they may be undefining a predefined macro
+    NewMacros.push_back(std::move(PPOpts.Macros[OriginalIndex]));
+  }
+  std::swap(PPOpts.Macros, NewMacros);
+}
+
 /// A clang tool that runs the preprocessor in a mode that's optimized for
 /// dependency scanning for the given compiler invocation.
 class DependencyScanningAction : public tooling::ToolAction {
@@ -203,6 +275,8 @@ class DependencyScanningAction : public tooling::ToolAction {
     CompilerInvocation OriginalInvocation(*Invocation);
     // Restore the value of DisableFree, which may be modified by Tooling.
     OriginalInvocation.getFrontendOpts().DisableFree = DisableFree;
+    if (any(OptimizeArgs & ScanningOptimizations::Macros))
+      canonicalizeDefines(OriginalInvocation.getPreprocessorOpts());
 
     if (Scanned) {
       // Scanning runs once for the first -cc1 invocation in a chain of driver

clang/test/ClangScanDeps/optimize-canonicalize-macros.m

@@ -0,0 +1,87 @@
+// This test verifies that command lines with equivalent -D and -U arguments
+// are canonicalized to the same module variant.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: sed -e "s|DIR|%/t|g" %t/build/compile-commands.json.in > %t/build/compile-commands.json
+// RUN: clang-scan-deps -compilation-database %t/build/compile-commands.json \
+// RUN:   -j 1 -format experimental-full -optimize-args=canonicalize-macros > %t/deps.db
+// RUN: cat %t/deps.db | sed 's:\\\\\?:/:g' | FileCheck %s -DPREFIX=%/t
+
+// Verify that there are only two variants and that the expected merges have
+// happened.
+
+// CHECK:      {
+// CHECK-NEXT:   "modules": [
+// CHECK-NEXT:     {
+// CHECK-NEXT:       "clang-module-deps": [],
+// CHECK-NEXT:       "clang-modulemap-file":
+// CHECK-NEXT:       "command-line": [
+// CHECK-NOT:          "J=1"
+// CHECK-NOT:          "J"
+// CHECK-NOT:          "K"
+// CHECK:            ],
+// CHECK-NEXT:       "context-hash": "{{.*}}",
+// CHECK-NEXT:       "file-deps": [
+// CHECK:            ],
+// CHECK-NEXT:       "name": "A"
+// CHECK-NEXT:     },
+// CHECK-NEXT:     {
+// CHECK-NEXT:       "clang-module-deps": [],
+// CHECK-NEXT:       "clang-modulemap-file":
+// CHECK-NEXT:       "command-line": [
+// CHECK:              "Fඞ"
+// CHECK:              "F\\u{0D9E}"
+// CHECK:              "K"
+// CHECK:              "K"
+// CHECK:            ],
+// CHECK-NEXT:       "context-hash": "{{.*}}",
+// CHECK-NEXT:       "file-deps": [
+// CHECK:            ],
+// CHECK-NEXT:       "name": "A"
+// CHECK-NEXT:     }
+// CHECK-NEXT:   ],
+// CHECK-NEXT:   "translation-units": [
+// CHECK:        ]
+// CHECK:      }
+
+
+//--- build/compile-commands.json.in
+
+[
+{
+  "directory": "DIR",
+  "command": "clang -c DIR/tu0.m -DJ=1 -UJ -DJ=2 -DI -DK(x)=x -I modules/A -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-module-maps",
+  "file": "DIR/tu0.m"
+},
+{
+  "directory": "DIR",
+  "command": "clang -c DIR/tu1.m -DK -DK(x)=x -DI -D \"J=2\" -I modules/A -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-module-maps",
+  "file": "DIR/tu1.m"
+},
+{
+  "directory": "DIR",
+  "command": "clang -c DIR/tu2.m -I modules/A -DFඞ '-DF\\u{0D9E}' -DK -DK -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-module-maps",
+  "file": "DIR/tu2.m"
+}
+]
+
+//--- modules/A/module.modulemap
+
+module A {
+  umbrella header "A.h"
+}
+
+//--- modules/A/A.h
+
+//--- tu0.m
+
+#include <A.h>
+
+//--- tu1.m
+
+#include <A.h>
+
+//--- tu2.m
+
+#include <A.h>

clang/tools/clang-scan-deps/ClangScanDeps.cpp

@@ -157,6 +157,7 @@ static void ParseArgs(int argc, char **argv) {
             .Case("header-search", ScanningOptimizations::HeaderSearch)
             .Case("system-warnings", ScanningOptimizations::SystemWarnings)
             .Case("vfs", ScanningOptimizations::VFS)
+            .Case("canonicalize-macros", ScanningOptimizations::Macros)
             .Case("all", ScanningOptimizations::All)
             .Default(std::nullopt);
     if (!Optimization) {